Exhaust emission control device

ABSTRACT

Provided is an exhaust emission control device which can properly burn off soot and soluble organic fraction attached to and accumulated on electrodes of a plasma generator. The exhaust emission control device with a post-processing device (catalyst regenerative particulate filter) for allowing exhaust gas to pass therethrough for gas purification incorporated in an exhaust pipe of an internal combustion engine (diesel engine) comprises a plasma generator arranged upstream of the post-processing device for discharging electricity in the exhaust gas to generate plasma, flow-through type oxidation catalyst arranged upstream of the plasma generator, fuel adding means (controller) arranged upstream of the oxidation catalyst for adding fuel into the exhaust gas, temperature increasing means (suction throttling valve or controller) for increasing the exhaust temperature to a level enough for oxidation reaction on the oxidation catalyst of the fuel added by said fuel adding means.

TECHNICAL FIELD

The present invention relates to an exhaust emission control device.

BACKGROUND ART

Particulates or particulate matter emitted or discharged from a dieselengine is mainly constituted by carbonic soot and a soluble organicfraction (SOF) of high-boiling hydrocarbons, and contains a trace ofsulfate (misty sulfuric acid fraction). In order to reduce emission ofparticulates from the engine, a particulate filter is conventionallyemployed and incorporated into an exhaust pipe through which the exhaustgas flows.

This kind of particulate filter is a porous honeycomb structure made ofceramics such as cordierite and having lattice-like compartmentalizedpassages; alternate ones of the passages having plugged inlets and theremaining passages having unplugged open inlets which are plugged attheir outlets. Thus, only the exhaust gas passing through thecompartment walls is discharged downstream.

The particulates in the exhaust gas, which are captured by andaccumulated on the compartment walls of the particulate filter, arerequired to be burned off to regenerate the particulate filter beforeexhaust resistance considerably increases due to clogging. However, theexhaust from the diesel engine in a normal operating status rarely has achance to reach a temperature level at which the particulates ignite bythemselves. Then, to employ a catalytic regenerative particulate filterhas been investigated which integrally carries oxidation catalyst madefrom platinum-carrying alumina or in which separate oxidation catalystis arranged upstream of the particulate filter.

Thus, such employment of the catalyst regenerative particulate filteraccelerates the oxidation reaction of the captured particulates andlowers ignition temperature thereof, whereby the particulates can beburned off at an exhaust temperature lower than ever before.

Other than the above-mentioned particulate filter, a post-processingdevice such as selective reduction or NO_(x)-occlusion reductioncatalyst for removal of NO_(x) in exhaust gas has been proposed to beincorporated in an exhaust pipe; in particular, recently, apost-processing device in the form of a combination of a particulatefilter with NO_(x)-occlusion reduction catalyst has been developed.

However, whenever any of these post-processing devices is employed, anexhaust temperature above a predetermined level is required for assuredburning-off of particulates and for obtaining enough catalyticactivities. Thus, if an operating status with lower exhaust temperature(generally speaking, a region with lower exhaust temperature extends ina light-load operating region) continues, the post-processing devicecannot work well and in a case of, for example, a city shuttle-bus whichtends to travel on congested roads, operation above a predeterminedtemperature requirement does not continue for a long time, resulting inthe possibility of insufficiently obtaining the exhaust emission controleffect due to provision of a post-processing device.

To overcome this problem, there has been investigated arranging a plasmagenerator upstream of the post-processing device so as to obtain enoughexhaust emission control effect due to the post-processing device evenin an operating region with lower exhaust temperature. To dischargeelectricity by such plasma generator in the exhaust gas upstream of thepost-processing device to generate plasma excites the exhaust gas andchanges unburned hydrocarbon, oxygen and NO into an activated radical,ozone and NO₂, respectively. Because of these exhaust gas excitedcomponents being active, exhaust emission control effect due to thepost-processing device can be obtained even in a region with exhausttemperature lower than ever before.

As to an exhaust emission control device with a plasma generatorarranged upstream of a particulate filter, the following Reference 1 isknown as the prior art.

[Reference 1] JP 2002-276333A

SUMMARY OF THE INVENTION

[Problems to be Solved by the Invention]

However, electrodes of such plasma generator are exposed to exhaust gasflow having the particulates entrained thereon, so that carbonic sootand SOF may be attached to and accumulated on the electrodes to causeleakage of current, resulting in difficulty in applying voltage acrossthe electrodes and in hindrance of the generation of plasma.

The present invention was made in view of the above and has as itsobject to provide an exhaust emission control device which can properlybum off soot and a soluble organic fraction attached to and accumulatedon electrodes of a plasma generator.

The invention also deals with arrangement of this kind of plasmagenerator upstream of a post-processing device and provides an optimumcontrivance structure which is directed to concurrent reduction inamount of particulates as well as of NO_(x) and which can attain assuredregeneration of a particulate filter and satisfactory NO_(x) reductioneffect even in a driving state with lower exhaust temperature.

[Means or Measure for Solving the Problems]

The invention is directed to an exhaust emission control device with apost-processing device for allowing exhaust gas to pass therethrough forgas purification incorporated in an exhaust pipe of an internalcombustion engine, characterized in that it comprises a plasma generatorarranged upstream of the post-processing device for dischargingelectricity into the exhaust gas to generate plasma, flow-through typeoxidation catalyst arranged upstream of the plasma generator, fueladding means arranged upstream of the oxidation catalyst for adding fuelin the exhaust gas and temperature increasing means for increasingexhaust temperature to a level enough for oxidation reaction on theoxidation catalyst of the fuel added by the fuel adding means.

Thus, to discharge electricity in the exhaust gas by the plasmagenerator excites the exhaust gas and changes unburned hydrocarbon,oxygen and NO into activated radical, ozone and NO₂, respectively, theseexhaust gas excited components being active to thereby obtain exhaustemission control effect due to the post-processing device even in aregion with exhaust temperature lower than ever before.

When the soot and SOF in the exhaust gas attached to and accumulated onthe electrodes of the plasma generator are to be removed, fuel is addedupstream of the oxidation catalyst by fuel adding means. This added fuelmakes oxidation reaction through oxidation catalyst to generate reactionheat which substantially increases the temperature of the exhaust gaspassing through the oxidation catalyst. As a result, the exhaust gasincreased in temperature through the oxidation catalyst is introducedinto the plasma generator, leading to burn-off of the soot and SOFattached to and accumulated on the electrodes of the plasma generator.

If operation is being conducted in an operating region with too low anexhaust temperature to oxidize the fuel on the oxidation catalyst, theexhaust temperature may be properly increased by the temperatureincreasing means before fuel is added by the fuel adding means.

On practicing the exhaust emission control device of the invention moreconcretely, preferably a temperature sensor for detecting the exhausttemperature is arranged between the oxidation catalyst and the plasmagenerator, fuel being added properly by the fuel adding means only on acondition that a detected value of the temperature sensor exceeds apredetermined threshold. When the detected value of the temperaturesensor is below the threshold, an increase in the temperature of theexhaust gas may be properly conducted by the temperature increasingmeans before the fuel addition by the fuel adding means.

In the invention, the fuel adding means is preferably fuel injectioncontrol means which may cause a fuel injection unit to conductpost-injection following the main injection and with non-ignition timingafter a compressive top dead center.

The temperature increasing means may be suction throttling means forproperly throttling the suction flow rate. Alternatively, it may be fuelinjection control means which may cause the fuel injection unit tocontrol the main injection with a timing delayed within a combustiblerange to a normal injection or which may cause the fuel injection unitto conduct post-injection with a combustible timing just after the maininjection.

When the temperature increasing means is the suction throttling meansfor properly throttling the suction flow rate, such throttling of thesuction flow rate by the suction throttling means in an operating statuswith lower exhaust temperature causes the working air quantity in theinternal combustion engine to be reduced to increase the pumping loss,whereby the exhaust temperature is increased by increasing the injectedfuel amount so as to generate any required output. Reduction ingenerated exhaust gas amount by the combustion in the internalcombustion engine and resultant lowering of the heat capacity contributeto further increase in the exhaust temperature.

When the temperature increasing means is the fuel injection controlmeans and the fuel injection unit is caused to conduct the maininjection with the timing delayed within the combustible range to thenormal injection, fuel of the delayed main injection is burned withtiming hardly convertible into output to lower the heat efficiency inthe internal combustion engine and increase the heat quantity not usedas motive energy in the heat release value of the fuel, therebyincreasing the exhaust temperature.

When the temperature increasing means is the fuel injection controlmeans and the fuel injection unit is caused to conduct post-injectionwith a combustible timing just after the main injection, the fuel of thepost-injection is burned with timing hardly convertible into output tolower the heat efficiency of the internal combustion engine and increasea heat quantity not used as motive energy among a heat release value ofthe fuel, thereby increasing the exhaust temperature.

It is preferable in the invention to provide judgment means fordetermining whether fuel addition is required or not through monitoringat least either of current and voltage upon generation of plasma by theplasma generator to judge any generation of leakage, which makes itpossible to avert any and all wasteful fuel addition.

When the catalyst regenerative particulate filter is incorporated as apost-processing device in the exhaust pipe, it is preferable that aNO_(x) reduction catalyst for reductive purification of NO_(x) in theexhaust gas is provided downstream of the particulate filter and that aplasma generator for generating plasma through electric discharge in theexhaust gas is provided upstream of the particulate filter, the plasmagenerator being actuated in an operating status with lower exhausttemperature.

In such a case, electric discharging in the exhaust gas by the plasmagenerator in an operating status with lower exhaust temperature excitesthe exhaust gas to generate active radical and change NO into NO₂;because of these exhaust gas excited components being active, anoxidation reaction of the particulates captured on the particulatefilter is accelerated by the exhaust gas excited components, whereby theparticulates are burned off well even in an operating status with lowerexhaust temperature.

Furthermore, the relatively stable exhaust gas excited components suchas NO₂ redundant from the oxidation reaction of the capturedparticulates flow into the downstream NO_(x) reduction catalyst as theyare, NO_(x) being efficiently reduced in the NO_(x) reduction catalyst.

When the NO_(x) reduction catalyst is for example NO_(x)-occlusionreduction catalyst, electric discharge by the plasma generator causesNO, which occupies the majority of NO_(x) in the exhaust gas, to bechanged into highly responsive NO₂ which flows into the NO_(x)-occlusionreduction catalyst; as a result, such NO₂ is effectively occluded in thestate of nitrate so that occlusion reaction of NO_(x) in theNO_(x)-occlusion reduction catalyst is remarkably accelerated, resultingin obtaining NO_(x) reduction effect higher than that obtained in a casewhere no plasma assist is conducted.

When the occluded NO_(x) is to be decomposed and discharged, for examplepost-injection may be conducted at the engine side to add fuel in theexhaust gas, which will lower the oxygen concentration in the exhaustgas and increase reduction components in the exhaust gas such asunburned hydrocarbon and CO to accelerate decomposition and discharge ofNO_(x).

In this case, the unburned hydrocarbon is reacted with oxygen throughthe oxidation catalyst on the upstream particulate filter to bethermally decomposed into CO and hydrogen; such increase of CO andhydrogen remarkably accelerates decomposition and discharge reaction ofNO_(x) from NO_(x)-occlusion reduction catalyst as well as reductivepurification reaction of NO_(x).

When the NO_(x) reduction catalyst is a selective reduction catalyst, NOwhich occupies the majority of NO_(x) in the exhaust gas is changed byelectric discharge by the plasma generator into highly reactive NO₂which flows to the selective reduction catalyst. Thus, when an additiondevice is arranged upstream of the selective reduction catalyst to add areducer such as urea into the exhaust gas, NO₂ is effectively reducedinto N₂ through selective reduction catalyst; as a result, obtained isNO_(x) reduction effect higher than that obtained in a case where noplasma assist is conducted.

In practicing the invention more concretely, it is preferable to providea temperature sensor for detecting the exhaust temperature and acontroller for actuating the plasma generator on the basis of thedetection signal from the temperature sensor when the exhausttemperature detected is below a predetermined value; the controller maybe constructed such that it may optimize generated plasma amountdepending upon the exhaust temperature upon actuation of the plasmagenerator.

[Effects of the Invention]

According to the above-mentioned exhaust emission control device of theinvention, the following various excellent meritorious effects will beobtained:

-   (I) According to an aspect of the invention, after the exhaust    temperature may be increased as needs demand by temperature    increasing means, fuel is added to the exhaust gas by the fuel    adding means. The added fuel is oxidized through the oxidation    catalyst and a resultant reaction heat substantially increases the    temperature of the exhaust gas passing through the oxidation    catalyst. This exhaust gas is introduced into the plasma generator    to burn off the soot and SOF attached to and accumulated on the    electrodes, so that preliminarily prevented is leakage of the    current due to the soot and SOF attached to and accumulated on the    electrodes, whereby proper voltage is applied with no hindrance    across the electrodes to maintain well generation of plasma.-   (II) On the basis of the detected value of the temperature sensor,    the fuel adding means and temperature increasing means can be    properly operated to efficiently burn off the soot and SOF attached    to the electrodes of the plasma generator.-   (III) Merely conducted is controlling the fuel injection unit so as    to cause it to conduct post-injection following the main injection    and with non-ignition timing later than a compressive top dead    center, which make it possible to add unburned fuel to the exhaust    gas with no need of new facilities annexed, thereby suppressing a    runup of cost on fuel adding means.-   (IV) The working air quantity in the internal combustion engine is    reduced to increase pumping loss; and exhaust gas to be generated in    the combustion in the internal combustion engine is reduced to lower    the heat capacity. As a result, the temperature of the exhaust gas    to the oxidation catalyst can be surely increased.-   (V) Fuel of the delayed main injection is burned with timing hardly    convertible into output, so that the heat efficiency of the internal    combustion engine is lowered to increase heat quantity not utilized    for motive energy among the heat release value of the fuel, whereby    the temperature of the exhaust gas to the oxidation catalyst can be    surely increased.-   (VI) The fuel of the post-injection is burned with timing hardly    convertible to output, so that the heat efficiency of the internal    combustion engine is lowered to increase heat quantity not utilized    as motive energy among heat release value of the fuel, whereby the    temperature of the exhaust gas to the oxidation catalyst can be    surely increased.-   (VII) Any and all wasteful fuel addition is averted so that cost for    fuel addition can be suppressed to requisite minimum.-   (VIII) According to a further aspect of the invention, a particulate    filter is reliably regenerated even in an operating status with    lower exhaust temperature and good NO_(x) reduction effect is    obtained by the NO_(x) reduction catalyst; moreover, any and all    undue wasteful plasma generation is averted to substantially    suppress electricity consumption.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described with reference to thedrawings.

FIGS. 1 to 3 show an embodiment of the invention. In FIG. 1, referencenumeral 1 designates a diesel engine (internal combustion engine) with aturbocharger 2. Suction air 4 sucked through an air cleaner 3 is guidedvia a suction pipe 5 to a compressor 2 a of the turbocharger 2 so as tocompress the same, the compressed suction air 4 being passed through anintercooler 6 and being distributed to respective cylinders of thediesel engine 1.

Exhaust gas 8 discharged via an exhaust manifold 7 from the respectivecylinders of the diesel engine 1 is fed to a turbine 2 b of theturbocharger 2; the exhaust gas 8 having driven the turbine 2 b ispassed through a catalyst regenerative particulate filter 10(post-processing device) for capturing of particulates and isdischarged.

As shown in FIG. 2 on an enlarged scale, the particulate filter 10 is aporous honeycomb structure made of ceramics having lattice-likecompartmentalized passages 10 a; alternate ones of the passages 10 ahave plugged inlets and the remaining passages 10 a with unplugged openinlets are plugged at their outlets. Thus, only the exhaust gas 8passing through the porous compartment walls 10 b is dischargeddownstream.

Arranged upstream of the particulate filter 10 is a plasma generator 11which discharges electricity in the exhaust gas 8 to generate plasma.Arranged upstream of the plasma generator 11 is flow-through typeoxidation catalyst 12 with a honeycomb structure shown in an enlargedscale in FIG. 3.

The plasma generator 11 has electrodes 13 and 14 arranged oppositely toeach other for possible mutual electric discharge. The electrodes 13 and14 may be of any combination of any shapes such as plate, rod orcylinder, provided that mutual distances between them can be set to besubstantially uniform.

Each of the electrodes 13 and 14 is connected through an electricdischarge controller 15 to a power supply 16; particularly in thepresent embodiment, the power supply 16 is assumed to be an in-vehiclecell, so that the voltage of the power supply 16 is enhanced by thedischarge controller 15 to a voltage capable of discharging electricityand is supplied across the respective electrodes 13 and 14. Thedischarge controller 15 is controlled by a command signal 15 a from acontroller 17 which is an engine control computer or electronic controlunit (ECU).

Arranged between the oxidation catalyst 12 and the plasma generator 11is a temperature sensor 18 for detecting exhaust temperature, adetection signal 18 a from the sensor 18 being inputted to thecontroller 17.

The controller 17, which serves as ECU, also takes control of fuelinjection. More specifically, a fuel injection signal 21 a is outputtedto a fuel injection unit 21 which injects fuel to respective cylindersof the diesel engine 1 on the basis of an accelerator stepped-on degreesignal 19 a from an accelerator stepped-on degree sensor 19 (loadsensor) which detects an accelerator stepped-on degree as a load to thediesel engine 1 as well as an engine revolution speed signal 20 a from arevolution speed sensor 20 which detects revolution speed of the engine1.

The fuel injection unit 21 comprises a plurality of injectors each foreach of the cylinders, an electromagnetic valve of each of the injectorsbeing controlled for valve opening by the fuel injection signal 21 a sothat fuel injection timing (valve opening timing) and fuel injectedamount (valve opening time) are properly controlled.

In the controller 17, a fuel injection signal 21 a in a normal mode isdetermined on the basis of an accelerator stepped-on degree signal 19 aand revolution speed signal 20 a. When fuel addition by post-injectionis required, the fuel injection signal 21 a is determined such thatchangeover is effected from the normal mode to electrode regenerationmode where post-injection is conducted with non-ignition timing followedby the main injection conducted near a compressive top dead center(crank angle of 0°).

Thus, when the post-injection is conducted with non-ignition timinglater than the compressive top dead center after the main injection,this post-injection adds unburned fuel (mainly, hydrocarbon) in theexhaust gas 8, such unburned fuel being oxidized through the oxidationcatalyst 12 to generate reaction heat which substantially increases thetemperature of the exhaust gas 8 passing through the oxidation catalyst12.

The controller 17 serves for conducting fuel addition in the form ofpost-injection, based on the detection signal 18 a from the temperaturesensor 18, only in a condition that the temperature (predeterminedthreshold) which enables oxidation reaction on the oxidation catalyst 12of fuel added by the post-injection is exceeded. If not, fuel additionin the form of the post-injection is conducted after the exhausttemperature is increased by the temperature increasing means (referredto hereinafter).

More specifically, in the example illustrated, an action is generated bythe opening degree command signal 22 a from the controller 17 to thesuction throttling valve 22 (suction throttling means) incorporated inthe suction valve 5, which action is different from an inherent actionfor the suction throttling valve 22 and is for utilization of the valveas temperature increasing means for increasing the exhaust temperature.When the throttling of the suction flow rate is conducted by the suctionthrottling valve 22 in the driving state with lower exhaust temperature,the working air quantity in the diesel engine 1 is reduced to increasepumping loss, whereby the exhaust temperature is increased by increasingthe injected fuel amount so as to generate any required output.Reduction in generated exhaust gas 8 amount by the combustion in thediesel engine 1 and resultant lowering of the heat capacity contributeto further increase in the exhaust temperature.

Alternatively, the controller 17, which serves also as fuel injectioncontrol means, may be utilized as the temperature increasing means forincreasing the exhaust temperature. More specifically, the fuelinjection unit 21 is caused to conduct the main injection with thetiming delayed in a combustible range by the controller 17 to the normalinjection; alternatively, the fuel injection unit 21 is caused toconduct post-injection with the combustible timing just after the maininjection.

More specifically, when the main injection is conducted with timingdelayed within the combustible range to the normal injection, fuel inthe delayed main injection is burned with a timing hardly convertibleinto an output so that the heat efficiency of the diesel engine 1 islowered and heat quantity not utilized as motive energy among the heatrelease value of the fuel is increased to increase the exhausttemperature.

When the post-injection is conducted with the combustible timing justafter the main injection, the fuel of the post-injection is burned witha timing hardly convertible into an output so that heat efficiency ofthe diesel engine 1 is lowered and heat quantity not utilized as motiveenergy among the heat release value of fuel is increased to increase theexhaust temperature.

The above-mentioned temperature increasing means is controlled on thebasis of the detection signal 18 indicative of whether exceeded is athreshold or critical temperature above which added fuel cannot beoxidized on the oxidation catalyst 12. When the threshold is notexceeded, the temperature increasing mode is conducted to actuate thetemperature increasing means before the electrode regeneration mode forconducting the post-injection; whereas, on the condition that thedetected value of the temperature sensor 18 exceeds the predeterminedthreshold, changeover is effected to the electrode regeneration modewhere the post-injection is conducted.

Thus, with the exhaust emission control device constructed in thismanner, electricity is discharged into the exhaust gas 8 by the plasmagenerator 11 to excite the exhaust gas 8 so that unburned hydrocarbon,oxygen and NO are changed into activated radical, ozone and NO₂,respectively. With these exhaust excited components being active, theoxidation reaction of particulates captured on the particulate filter 10is accelerated by the exhaust excited components, whereby theparticulates may be ignited to be burned off even with exhausttemperature lower than ever before.

When the soot and SOF entrained in the exhaust gas 8 are attached to andaccumulated on the electrodes 13 and 14 of the plasma generator 11 andthe soot and SOF attached thereon are to be removed, the electroderegeneration mode is selected on the condition that the detected valueof the temperature sensor 18 exceeds the predetermined threshold,; thatis, fuel injection pattern is changed over by the controller 17 from thenormal mode to the electrode regeneration mode so that adopted is aninjector pattern according to which post-injection follows the maininjection and with non-ignition timing later than a compressive top deadcenter. As a result, the fuel added by the post-injection to the exhaustgas 8 while being unburned is oxidized on the oxidation catalyst 12 togenerate reaction heat. This reaction heat substantially increases thetemperature of the exhaust gas 8 passing through the oxidation catalyst12; the exhaust gas 8 increased in temperature while passing through theoxidation catalyst 12 is introduced in the plasma generator 11 so thatthe soot and SOF attached to and accumulated on the electrodes 13 and 14of the plasma generator 11 are burned off.

Even if the operation is conducted in an operating region with too lowan exhaust temperature to oxidize the added fuel on the oxidationcatalyst 12, by the controller 17 which receives the detection signal 18a from the temperature sensor 18, the temperature increasing mode isconducted before the electrode regeneration mode, so that the suctionthrottling valve 22 is throttled to increase the temperature of theexhaust gas 8 passing to the oxidation catalyst 12. Alternatively, themain injection may be delayed within a combustible range to the ordinaryinjection or post-injection may be conducted with a combustible timingjust after the main injection of the fuel.

When the detected value of the temperature sensor 18 exceeds thepredetermined threshold and changeover is effected to the electroderegeneration mode, the added fuel can be surely oxidized on theoxidation catalyst 12. The exhaust gas 8 substantially increased intemperature by the reaction heat bums off the soot and SOF attached toand accumulated on the electrodes 13 and 14 of the plasma generator 11.

As to whether soot and SOF attached to the electrodes 13 and 14 of theplasma generator 11 are recquired to be removed or not, for example,voltage and/or current upon plasma generation by the plasma generator 11may be always monitored by the controller 17 as the judging means todetermine whether the leakage is generated or not. Alternatively,post-injection may be conducted regularly on the basis of for exampleoperating time.

Thus, according to the above-mentioned embodiment, the exhausttemperature may be increased by the temperature increasing means such asa suction throttling valve 22 as needs demand and fuel is added to theexhaust gas 8 by post-injection. The added fuel is oxidized on theoxidation catalyst 12 so that the resultant reaction heat substantiallyincreases the temperature of the exhaust gas 8 passing through theoxidation catalyst 12. The exhaust gas 8 is introduced into the plasmagenerator 11 to bum off the soot and SOF attached to and accumulated onthe electrodes 13 and 14, so that preliminarily prevented is leakage ofcurrent due to the soot and SOF attached and accumulated, whereby propervoltage is applied with no hindrance across the electrodes 13 and 14 tomaintain well generation of plasma.

FIGS. 4-7 show a further embodiment of the invention wherein a catalystregenerative particulate filter 10 is incorporated as post-processingdevice into an exhaust pipe 9. Arranged downstream of the particulatefilter 10 is flow-through type NO_(x)-occlusion reduction catalyst 23 asNO_(x) reduction catalyst for reductive purification of NO_(x) in theexhaust gas 8 (for example, alumina catalyst carrying platinum andbarium or alumina catalyst carrying iridium, platinum and barium isknown as this kind of NO_(x)-occlusion reduction catalyst 23). Arrangedupstream of the particulate filter 10 is a plasma generator 11 similarto that shown in the embodiment of FIGS. 1 to 3.

More specifically, the plasma generator 11 is actuated by the electricdischarge controller 15 which receives the command signal 15 a from thecontroller 17. Inputted into the controller 17 is the detection signal18 a from the temperature sensor 18 which detects the exhausttemperature at an entry side of the plasma generator 11. The plasmagenerator 11 is actuated on the basis of the detection signal 18 a whenthe exhaust temperature is below the predetermined value.

However, actuation of the plasma generator 11 with the exhausttemperature being below the predetermined value is not necessarilyrequisite. For example, pressure loss of the particulate filter 10 isdetected by a pressure sensor to determine a particulate accumulatedamount and, only on an occasion that the accumulated amount is judged tobe much, the plasma generator 11 may be activated with the exhausttemperature below the predetermined value. The accumulated particulateamount may be momentarily determined by calculation from generated anddealt amounts assumed on the basis of the operating status, or may bejudged on the basis of the operating time.

Moreover, particularly in the present embodiment, when the plasmagenerator 11 is actuated, the generated plasma amount is optimizeddepending upon the exhaust temperature; more specifically, as shown inthe graph of FIG. 5, optimization is carried out in the controller 17 byadjusting voltage, current or frequency so as to increase the generatedplasma amount as the exhaust temperature lowers relative to thepredetermined value x, whereby superfluous or undue plasma generation issuppressed to suppress electricity consumption to requisite minimum.

Thus, in the operating status with lower exhaust temperature, thecommand signal 15 a is outputted from the controller 17 on the basis ofthe detection signal 18 a from the temperature sensor 18. The electricdischarge controller 15 which receives the command signal 15 a causesthe plasma generator 11 to be actuated to discharge electricity in theexhaust gas 8 so that the exhaust gas 8 is excited to generate activeradical and change NO into NO₂; because of these exhaust gas excitedcomponents being active, the oxidation reaction of the particulatescaptured on the particulate filter 10 is accelerated by the exhaust gasexcited components, whereby the particulates are satisfactorily burnedoff even in the operating status with lower exhaust temperature.

In fact, verification experiments by the inventor revealed that, asshown in solid line in the graph of FIG. 6, when no plasma assist (noelectric discharge by the plasma generator 11) is conducted, at least230° C. or so of exhaust temperature is required for combustion of thecaptured particulates; as shown in dotted line in the graph of FIG. 6,when plasma assist is conducted, even the exhaust temperature lower than230° C. will suffice for burning-off of the captured particulates withrequired combustion rate.

Relatively stable exhaust gas excited components such as NO₂ redundantfrom the oxidation reaction of the captured particulates on theparticulate filter 10 flow to the downstream NO_(x)-occlusion reductioncatalyst 23 as they are, whereby reduction of NO_(x) is effected on theNO_(x)-occlusion reduction catalyst 23.

More specifically, electric discharge by the plasma generator 11 changesNO, which occupies the majority of NO_(x) in the exhaust gas 8, intohighly reactive NO₂ which flows to the NO_(x)-occlusion reductioncatalyst 23, so that NO₂ is efficiently occluded in the form of nitrite.As a result, occlusion reaction of NO_(x) on the NO_(x)-occlusionreduction catalyst 23 is remarkably accelerated, whereby obtained isNO_(x) reduction effect higher than that in a case where no plasmaassist is effected.

In fact, verification experiments by the inventor revealed that, asshown in solid line in the graph of FIG. 7, when no plasma assist isconducted, at least 250° C.-300° C. of exhaust temperature is requiredfor development of NO_(x) reduction ratio; as shown in dotted line inthe graph of FIG. 7, when plasma assist is conducted, NO_(x) reductionratio is developed even at considerably low exhaust temperaturesubstantially less than 250° C.-300° C.

In order to decompose and discharge the occluded NO_(x), post-injectionor the like may be effected on the side of diesel engine 1 to add fuelin the exhaust gas 8, which lowers the oxygen concentration in theexhaust gas 8 and increases the reduction components in the exhaust gas8 such as unburned hydrocarbon and CO, thereby accelerating thedecomposition and discharge of NO_(x).

In this case, unburned hydrocarbon is reacted with oxygen on theoxidation catalyst carried by the upstream particulate filter 10 to bethermally decomposed, thereby generating CO and hydrogen. Such increaseof CO and hydrogen remarkably accelerates decomposing and dischargingreactions of NO_(x) from the NO_(x)-occlusion reduction catalyst 23 aswell as reductive purification reaction of NO_(x).

Thus, according to the above-mentioned embodiment, the plasma generator11 is actuated in the operating status with lower exhaust temperature,so that electric discharge by the plasma generator 11 generates highlyactive exhaust gas excited component in the exhaust gas 8; these exhaustexcited components remarkably accelerates the oxidation reaction of thecaptured particulates as well as NO_(x) occlusion reaction on theNO_(x)-occlusion reduction catalyst 23, so that even in the operatingstatus with lower exhaust temperature, the particulate filter 10 can besurely regenerated and favorable NO_(x) reduction effect can be obtainedby the NO_(x)-occlusion reduction catalyst 23.

The plasma generator 11 is actuated only in the operating status withlower exhaust temperature. Moreover, at such actuation, the generatedplasma amount is optimized depending upon exhaust temperature. As aresult, any and all superfluous and unduly plasma generation is avertedto substantially suppress the electricity consumption.

In the above-mentioned embodiment with respect to FIGS. 4-7,NO_(x)-occlusion reduction catalyst 23 has been adopted as the NO_(x)reduction catalyst. In lieu of this NO_(x)-occlusion reduction catalyst23, selective reduction catalyst with elevated response selectivitybetween urea and NO_(x) may be employed; then, electric discharge by theplasma generator 11 causes NO occupying the majority of NO_(x) in theexhaust gas 8 to be changed into highly reactive NO2 which flows to theselective reduction catalyst. Thus, the addition of urea into theexhaust gas 8 by for example an urea adding device (not shown) arrangedupstream of the selective reduction catalyst causes NO₂ to beeffectively reduced into N₂ with the urea being utilized as reducingagent, so that obtained is NO_(x) reduction effect higher than that in acase where no plasma assist is conducted.

In fact, verification experiments by the inventor revealed that, asshown in solid line in the graph of FIG. 8, when no plasma assist isconducted, at least 200° C.-250° C. or so of exhaust temperature isrequired for development of NO_(x) reduction ratio; as shown in dottedline in the graph of FIG. 8, when plasma assist is conducted, NO_(x)reduction ratio is developed even considerably lower exhaust temperaturesubstantially less than 200° C.-250° C.

Therefore, even in a case where selective reduction catalyst is employedas NO_(x) reduction catalyst, plasma assist by the plasma generator 11results in favorable NO_(x) reduction effect, whereby effects andadvantages similar to those obtained in the above embodiments can beobtained.

INDUSTRIAL APPLICABILITY

It is to be understood that an exhaust emission control device accordingto the invention is not limited to the above-mentioned embodiments andthat various changes and modifications may be made within the gist ofthe invention. For example, with respect to the embodiment shown inFIGS. 1-3, other than the catalyst regenerative particulate filter,selective reduction catalyst or NO_(x)-occlusion reduction catalyst forremoval of NO_(x) in the exhaust gas may be adopted as thepost-processing device. The fuel adding means may be in the form of aninjector passing through an exhaust pipe at an appropriate positionthereof (alternatively, it may pass through an exhaust manifold), thefuel being directly injected by the injector into the exhaust gas foraddition of the fuel to the exhaust gas. The three alternativetemperature increasing means disclosed in the above-mentionedembodiments may be employed singly or any combination thereof. Withrespect to the embodiments shown in FIGS. 4-7 and FIG. 8, in lieu ofdirect measurement of exhaust temperature, number of revolutions andload of an engine are detected to presume an operating status; and theplasma generator may be operated in an operating status presumed to bein an operating region with lower exhaust temperature. When selectivereduction catalyst is adopted as NO_(x) reduction catalyst, a reducingagent other than urea may be employed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an embodiment of the invention;

FIG. 2 is a sectional view showing particulars of a particulate filterin FIG. 1;

FIG. 3 is a perspective view partly cut out showing particulars ofoxidation catalyst in FIG. 1;

FIG. 4 is a schematic view showing a further embodiment of theinvention;

FIG. 5 is a graph showing a relationship between exhaust temperature andgenerated plasma amount.

FIG. 6 is a graph showing a relationship between exhaust temperature andcombustion rate of captured particulates.

FIG. 7 is a graph showing a relationship between exhaust temperature andNO_(x) reduction ratio with NO_(x)-occlusion reduction catalyst.

FIG. 8 is a graph showing a relationship between exhaust temperature andNO_(x) reduction ratio with selective reduction catalyst.

1. An exhaust emission control device with a post-processing device forallowing exhaust gas to pass therethrough for gas purificationincorporated in an exhaust pipe of an internal combustion engine,comprising a plasma generator arranged upstream of the post-processingdevice for discharging electricity into the exhaust gas to generateplasma, flow-through type oxidation catalyst arranged upstream of theplasma generator, fuel adding means arranged upstream of the oxidationcatalyst configured for adding fuel in the exhaust gas and temperatureincreasing means configured for increasing exhaust temperature to alevel enough for oxidation reaction on the oxidation catalyst of thefuel added by the fuel adding means; and a temperature sensor arrangedbetween the oxidation catalyst and the plasma generator for detectingexhaust temperature, fuel being added properly by the fuel adding meansonly on a condition that a detected value of the temperature sensorexceeds a predetermined threshold, the temperature of the exhaust gasbeing increased by the temperature increasing means before the fueladdition by the fuel adding means on a condition that the detected valeof the temperature sensor is below the predetermined threshold.
 2. Theexhaust emission control device according to claim 1, wherein the fueladding means comprises fuel injection control means which causes thefuel injection unit to conduct post-injection followed by the maininjection and with non-ignition timing later than a compressive top deadcenter.
 3. The exhaust emission control device according to claim 1,wherein the temperature increasing means for increasing the exhausttemperature comprises suction throttling means for properly throttlingsuction flow rate.
 4. The exhaust emission control device according toclaim 2, wherein the temperature increasing means for increasing theexhaust temperature comprises suction throttling means configured forproperly throttling suction flow rate.
 5. The exhaust emission controldevice according to claim 1, wherein the temperature increasing meansfor increasing the exhaust temperature is fuel injection controllingmeans configured for causing the fuel injection unit to conduct maininjection delayed within a combustible range to the normal injection. 6.The exhaust emission control device according to claim 2, wherein thetemperature increasing means for increasing the exhaust temperaturecomprises fuel injection controlling means configured for causing thefuel injection unit to conduct main injection delayed within acombustible range to the normal injection.
 7. The exhaust emissioncontrol device according to claim 1, wherein the temperature increasingmeans for increasing the exhaust temperature comprises fuel injectioncontrolling means configured for causing the fuel injection unit toconduct post injection with a combustible timing just after the maininjection.
 8. The exhaust emission control device according to claim 2,wherein the temperature increasing means for increasing the exhausttemperature comprises fuel injection controlling means configured forcausing the fuel injection unit to conduct post injection with acombustible timing just after the main injection.
 9. The exhaustemission control device according to claim 1, further comprising judgingmeans configured for determining whether fuel addition is required ornot through monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 10. The exhaust emissioncontrol device according to claim 2, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 11. The exhaust emissioncontrol device according to claim 3, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 12. The exhaust emissioncontrol device according to claim 4, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 13. The exhaust emissioncontrol device according to claim 5, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 14. The exhaust emissioncontrol device according to claim 6, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 15. The exhaust emissioncontrol device according to claim 7, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.
 16. The exhaust emissioncontrol device according to claim 8, further comprising judging meansconfigured for determining whether fuel addition is required or notthrough monitoring at least either of current and voltage upongeneration of plasma in the plasma generator.